Cation transport in lithium sulphate based crystals

Abstract

Molecular dynamics simulations have been carried out of the fast-ion-conducting solid phases of lithium sulphate (Li₂SO₄), sodium-doped Li₂SO₄ and lithium sodium sulphate (LiNaSO₄). An interionic potential model has been developed which provides a good fit to the experimental pressures, sound velocities, neutron structure factors and cation diffusion coefficients. The static distribution of the mobile cations is discussed in terms of the occupancy of interstitial sites and of anion-cation coordination. It is shown that the cations are almost invariably bonded to one and often to two oxygen atoms of neighbouring sulphate groups. The latter therefore play an essential role in the cation diffusion mechanism. The diffusion coefficients are partitioned in a way that makes it possible to identify separately the contributions from centre-of-mass displacements and rotations of the sulphate groups. It is concluded that the diffusion coefficient of Li⁺ is enhanced by up to approximately 50% by anion reorientation and that of Na⁺ by approximately 30%. The near equality of the diffusion coefficients of Li⁺ and Na⁺ in both doped Li₂SO₄ and LiNaSO₄ is shown to arise as a balance between mass and size effects; the implications of this for related systems are noted.